1. Field of the Invention
The present invention relates generally to polymeric materials, and more specifically to halogenated polymeric materials useful in the construction of devices for telecommunications.
2. Technical Background
In optical communication systems, messages are transmitted by electromagnetic carrier waves at optical frequencies that are generated by such sources as lasers and light-emitting diodes. There is interest in such optical communication systems because they offer several advantages over conventional communication systems.
One preferred device for routing or guiding waves of optical frequencies from one point to another is an optical waveguide. The operation of an optical waveguide is based on the fact that when a light-transmissive medium is surrounded or otherwise bounded by an outer medium having a lower refractive index, light introduced along the axis of the inner medium substantially parallel to the boundary with the outer medium is highly reflected at the boundary, trapping the light in the light transmissive medium and thus producing a guiding effect along the longitudinal axis of the inner medium. A wide variety of optical devices can be made which incorporate such light guiding structures as the light transmissive elements. Illustrative of such devices are planar optical slab waveguides, channel optical waveguides, rib waveguides, optical couplers, optical splitters, optical switches, optical filters, arrayed waveguide gratings, waveguide Bragg gratings, and variable attenuators. For light of a particular frequency, optical waveguides may support a single optical mode or multiple modes, depending on the dimensions of the inner light guiding region and the difference in refractive index between the inner medium and the surrounding outer medium.
Optical waveguide devices and other optical interconnect devices may be constructed from organic polymeric materials. While optical devices built from planar waveguides made of glass are relatively unaffected by temperature, devices made from organic polymers may show a significant variation of properties with temperature. This is due to the fact that organic polymeric materials have a relatively high thermo-optic coefficient (dn/dT). Thus, as an organic polymer undergoes a change in temperature, its refractive index changes appreciably. This property can be exploited to make active, thermally tunable or controllable devices incorporating light transmissive elements made from organic polymers. One example of a thermally tunable device is a 1×2 switching element activated by the thermo-optic effect. Thus, light from an input waveguide may be switched between two output waveguides by the application of a thermal gradient induced by a resistive heater. Typically, the heating/cooling processes occur over the span of one to several milliseconds.
Most polymeric materials, however, contain carbon-hydrogen bonds which absorb strongly in the 1550 nm wavelength range that is commonly used in telecommunications applications, causing devices made from such materials to have unacceptably high insertion losses. By lowering the concentration of C—H bonds in a material by replacement of C—H bonds with C—D or C-halogen bonds, it is possible to lower the absorption loss at infrared wavelengths. While planar waveguides made from fluorinated polyimides and deuterated or fluorinated polymethacrylates have achieved single mode losses of as little as 0.10 dB/cm at 1300 nm, it is relatively difficult to make optical devices from these materials. For example, the processes by which these waveguides have typically been made includes the use of a reactive ion etching process, which is cumbersome and can cause high waveguide loss due to scattering. Further, deuteration is not an effective means of reducing loss in the 1550 nm wavelength range. Fluorinated polyimides and deuterated or fluorinated polymethacrylates have higher losses in the telecommunications window near 1550 nm, typically on the order of 0.6 dB/cm. O—H and N—H bonds also contribute strongly to loss at wavelengths near 1310 nm and 1550 nm. Consequently, compositions are sought in which the concentrations of O—H and N—H bonds are minimal.
Photopolymers have been of particular interest for optical interconnect applications because they can be patterned using standard photolithographic techniques. Photolithography involves the selective polymerization of a layer of the photopolymer by exposure of the material to a pattern of actinic radiation. Material that is exposed to the actinic radiation is polymerized, whereas material that is not exposed remains unpolymerized. The patterned layer is developed, for example, by removal of the unexposed, unpolymerized material by an appropriate solvent.